1. Field of the Invention
This invention relates to a photothermographic material, especially suited for the manufacture of printing plates.
2. Prior Art
Photothermographic materials which are processed by a photothermographic process to form photographic images are disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969. The silver system of this type is generally known as a dry silver system.
These photothermographic materials generally contain a reducible silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), a toner for controlling the tonality of silver, and a reducing agent, typically dispersed in a binder matrix. Photothermographic materials are stable at room temperature. When they are heated at an elevated temperature (e.g., 80.degree. C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the organic silver salt in exposed regions provides black images in contrast to unexposed regions, eventually forming an image.
Such photothermographic materials have been used as microphotographic and radiographic photosensitive materials. However, only a few have been used as a graphic printing photosensitive material because the image quality is poor for the printing purpose as demonstrated by low maximum density (Dmax) and soft gradation.
With the recent advance of lasers and light-emitting diodes, scanners and image setters having an oscillation wavelength of 600 to 800 nm find widespread use. There is a strong desire to have a high contrast photosensitive material which has so high sensitivity and Dmax that it may comply with such output devices. Also a need for easy and dry processing is increasing.
U.S. Pat. No. 5,464,738 describes that high contrast images are obtainable using sulfonyl hydrazide as a reducing agent for dry silver. However, development does not take place unless the developing temperature is raised as high as 136.degree. C. to 142.degree. C.
U.S. Pat. No. 5,496,695 describes that high contrast images are obtainable using hindered phenol and formylhydrazine or tritylhydrazine as a reducing agent for dry silver. Such a combination of reducing agents still requires as high a temperature as 121 to 138.degree. C. (250 to 280.degree. F.) in order to produce high contrast images.
In the prior art photothermographic recording materials, polyethylene terephthalate (PET) film is commonly used as a support. The PET film is characterized by toughness, low moisture absorption and transparency. When heat development is carried out on a photothermographic recording material having a PET film support at a temperature of about 120.degree. C., the PET film undergoes shrinkage at a factor of more than about 0.1% although the exact shrinkage factor depends on manufacturing conditions of the PET film. Shrinkage of this order gives rise to no problem in prior art photothermographic recording materials because images produced therein are of low contrast.
One of technical problems encountered in the dry processing of a printing plate manufacturing system is to enhance the contrast of images. One solution to this problem is proposed in the above-referred U.S. Pat. No. 5,496,695. Since sharp dot images with a high black density were obtained due to achievement of super-high contrast, a possibility to use the photothermographic material as an intermediate material for the production of printed matter of quality was expected. In particular, the photothermographic material was desired to comply with a new printing plate technique of fabricating a high precision, high density screen. However, a serious problem was found. A false setting of color registration in color printing which remained less prominent in prior art photothermographic materials became so prominent that the printed matter appeared unacceptable. Thermal shrinkage largely differs between longitudinal and transverse directions of film and depending on the thermal hysteresis after manufacture, that is, thermal shrinkage is not constant. If four plates of Y, M, C and B are fabricated from sharp dot, super-high contrast photothermographic material, then a visually perceivable false setting of color registration can occur with conventional PET film.
A new support substitute for the conventional PET film support is thus desired for super-high contrast photothermographic material.
The above-mentioned image forming processes are characterized by heat development at a high temperature of 120.degree. C. or higher and suffer from several problems associated with formation of high contrast images. A first problem is noise known as pepper fog occurring when hydrazines are used. While black pepper is a phenomenon well known for conventional wet development using hydrazines, pepper fog occurring in a dry photothermographic system is considerably different from the black pepper of the wet development system in that the occurrence of pepper fog largely depends on heat development temperature and becomes more frequent at higher temperature. A second problem is a development variation. Particularly when a dot image is produced, the percent dot area irregularly varies within a single sheet of film. A third problem is that heat development causes a plastic film support to irregularly deform, losing flatness. These problems are serious as an intermediate material for the manufacture of printing plates. It is desired to solve these problems.
It is also desired to improve the storage stability and feed of such photothermographic material.
As a photosensitive material having high sensitivity, Dmax and contrast, we invented a photothermographic material comprising an organic silver salt, silver halide, developing agent and hydrazine derivative as claimed in Japanese Patent Application No. 228627/1995. This material has high sensitivity, Dmax and contrast enough to apply to printing photosensitive material. However, when it is desired to output images of more than 175 lines/inch so as to comply with the high precision printing technique which is increasingly demanded in the recent years, the support undergoes shrinkage or expansion by the heat during heat development, resulting in a false setting of color registration. Then the material can not be used in color printing application.
A system containing an organic silver salt, silver halide and developing agent, but free of hydrazine does not find use in high precision color printing application because of poor dot quality. The shrinkage of the support by heat development has never been considered a problem.
As compared with wet photosensitive material, photothermographic material using an organic solvent is extremely weak to external forces as by scratching because the adhesion between a coating and a support is poor. By selecting a binder or adding an adhesion modifier, the adhesion can be improved, but to a less extent. No effective measure for improving such adhesion is available.
In the printing field, ultrahigh photographic properties are desired. For example, U.S. Pat. No. 5,496,695 proposes to use hydrazine derivatives to accomplish ultrahigh properties. The use of hydrazine derivatives, however, gives rise to an image enlargement phenomenon that images are thickened due to infectious development, adversely affecting image reproducibility. An improvement in this regard is desired.